1. Field of the Invention
The present invention relates to an improvement of techniques for constructing a circuit for detecting a center level to correct same necessary to demodulate a FSK (Frequency Shift Keying) signal.
2. Description of the Related Art
In a demodulated output of a FSK signal there is an error in a center level due to various factors, in general, such as a frequency error of a transmitter and a receiver, a frequency error of a frequency discriminator and drift of a d.c. circuit system, etc. For this reason, various methods and circuits for detecting this center level to correct its center error have been conceived and used.
As representatives thereof there are known (1) a method for obtaining a center level of a demodulated base band signal of the FSK signal from an average of peak values held in a positive and a negative peak hold circuit; (2) a method in which there are disposed a positive and a negative dead zone circuit having dead zone voltage widths, which are in accordance with a positive and a negative peak value width, respectively of the demodulated base band signal, and the center error is detected from an output from said dead zone circuit exceeding these dead zone voltage widths in said base band signal; (3) a method for obtaining a center level by integrating a bit synchronizing signal at a beginning of the packet signal over a 2 bit length; (4) a method for obtaining a center level from an average of these sampled values obtained by sampling the bit synchronizing signal twice with an internal of 1 baud sec; etc.
However these methods have disadvantages as described below. That is, since, firstly, anyone of them obtains a center level, an error of the center level can be corrected by using it but there is a problem in processing thereafter. If the center level varies thereafter, anyone of methods of (1), (3) and (4) has no following ability with respect to variation of the center level.
According to the method of (2), when said signal varies between “1” and “0” frequently, an error in the center level can be corrected at any time because deviation of the signal from the dead zone can be detected. However, if said signal keeps to have “1” or “0”, variation of the center level from “1” or “0” can be detected when it deviates from the dead zone in a direction over said zone but said variation can not be detected when it deviates said zone in a reverse direction.
As is described above, in demodulation of a FSK signal even if the FSK signal keeps to have “1” or “0” at start of communication or during communication, a circuit capable of detecting a center level and correcting an error thereof always has not developed yet.